VARIATION 119 



not know how the variations have been brought about, 

 though he can often reason from analogy. Among 

 birds and mammals, especially, it has become custom- 

 ary to recognize subspecies. A subspecies is a phase or 

 form which is reasonably true to type within a given 

 area, but at one or more points intergrades with its 

 allies occupying adjacent territory. As Beebe has 

 shown in the case of birds, the peculiarity (e.g., a 

 darker or lighter color) may be due to the immediate 

 effects of environment, and the intergradation may be 

 merely the expression of the intergrading climatic condi- 

 tions. On the other hand, Sumner, experimenting with 

 subspecies of wild mice, has found genuine hereditary 

 differences. Mere inspection would not show which 

 kind of "subspecies" we were dealing with. Suppose 

 the differences to be inherited, the intergradation where 

 two types meet may be due to hybridization. Ento- 

 mologists recognize varieties and aberrations. The 

 aberration or "sport" is supposed to occur occasionally, 

 here and there. It may be known only by a single 

 specimen, though the species to which it belongs is 

 common. It is found, however, that the same kind of 

 difference may distinguish an aberration in one place, 

 and a local race or subspecies in another; and exactly 

 the same thing is true of plants. Botanists use the 

 word "form" or "forma" to designate minor varieties, 

 but with no regard to their genetic significance. Ulti- 

 mately the nomenclature of varieties will have to be 

 revised in the light of genetic research, but it is not 

 possible to do this thoroughly at present. 



